The system clock is the electronic circuit in a computer that generates a steady stream of tiny pulses--the digital signals that synchronize every operation. The system clock signal typically operates at a specific frequency between 50 million and 150 million cycles per second (megahertz, or MHz) and is precisely set by a quartz signal inside the clock circuit. The clock rate of a computer is one of the prime determinants of its overall processing speed, and it can go as high as the other components of the computer allow.
In order to achieve high performance, modern microprocessors often generate or synthesize on-chip clocking by multiplying the frequency of an off-chip source. Thus, a 50 MHz off-chip source can be used to generate on-chip clocking of 200 MHz for instance. The clock generation and distribution logic in a high performance microprocessor is sensitive and complex involving complex analog circuits, delay lines and feedback circuits, which are very sensitive to manufacturing process variations.
During manufacturing, and the debug and system testing, it is not practical to directly observe the on-chip clocking. Techniques to check the on-chip clocking during manufacturing are complex and time consuming and do not easily fit into the process of checking the digital circuitry. In order to reduce the test time for chips and to improve the quality of the testing of the clocking circuitry, a low cost but effective test is needed.